1. Field of the Invention
The present invention relates to a noise canceller which removes noise components from input signals. More particularly, it relates to a noise canceller which suppresses noise components in an input signal from an FM tuner or the like by predicting the value of the current digital signal based on past values.
2. Description of the Related Art
Recently, FM tuners which receive and demodulate FM broadcast waves have often been used aboard movable bodies. For example, car-mounted FM tuners are more susceptible to external pulse noise such as ignition noise produced by vehicles than typical FM tuners are. Therefore, in order to prevent degradation of sound quality caused by external noise, it is desirable that car-mounted FM tuners should be equipped with a noise canceller which removes noise components due to external noise, from detected signals.
To remove noise components from received FM signals, a commonly used configuration involves detecting noise components contained in detected signals or the like when the received FM signals are processed and holding the received FM signals during the duration of the noise components. Even if noise components are produced during reception of the FM signals, since this configuration holds the FM signals at the level at which they are at the start of noise generation and releases the hold when the noise subsides, it can reduce the effect of noise on the detected signals or the like sent out to downstream circuits.
Regarding a digital prediction filter for a noise canceller, Japanese Patent Laid-Open No. 5-22079, for example, discloses a configuration in which predicted values represented by a predictive equation are calculated by means of delay elements connected in multiple stages and a coefficient multiplier.
However, with a first noise canceller configured in the manner described above, received FM signals are held during periods in which noise components are generated during receiving FM waves, and thus sound signal components are removed together with noise components. For example, if external pulse noise such as ignition noise is generated during FM reception on a car-mounted FM tuner, sound signal components which correspond to a period equivalent to the pulse width of the external noise will be lost. This will deteriorate a distortion factor of the sound signal outputted after demodulation.
Also, FM signals received on an FM tuner generally consist of main signals such as the sound signal superimposed with various signals including a pilot signal for stereophonic broadcasting. For example, looking at a frequency distribution of detected signals, in addition to a 19-kHz pilot signal, various signals including a 38-kHz subcarrier are superimposed at integral multiples of 19 kHz in the frequency band up to 100 kHz. Therefore, digital FM tuners need to process various digital signals at a sufficiently high sampling frequency of at least 200 kHz or higher. Also, they are configured to supply the prediction filter described above with clock signals which correspond to the sampling frequency. Consequently, each of the delay elements which compose the prediction filter operates with a short delay time on the order of several microseconds. Thus, prediction time is on the order of several microseconds.
However, signal compensation based on predicted values produced by a prediction filter is not suited to deal with the above-mentioned ignition noise, which generally has a relatively large pulse width on the order of 50 microseconds.
Furthermore, if the prediction filter is configured to cover a frequency band up to 100 kHz to accommodate an FM tuner, errors are small at low frequencies, but increase sharply at high frequencies. FIG. 8 shows a frequency characteristic of a prediction filter when a linear predictive equation and a sampling frequency of 228 kHz are used. As shown in FIG. 8, prediction error is large around 19 kHz, which is the frequency of the pilot signal, and increases even more at integral multiples of 19 kHz. Consequently, when using a prediction filter in an FM tuner to remove noise, degradation in prediction accuracy for important signals such as the pilot signal and subcarrier becomes a problem.
The present invention has been achieved in view of the above problems. A first object of the present invention is to provide a noise canceller which can remove noise components due to external pulse noise introduced into the input signal while keeping original sound signal components intact, and thereby maintain good sound quality.
A second object of the present invention is to provide a noise canceller with an uncomplicated configuration which can determine predicted values with a high accuracy, even if external noise with a relatively large pulse width is introduced, and maintain high prediction accuracy when main signals are superimposed with various signals.